1. Field of the Invention
The present invention relates to a multibeam antenna arrangement comprising two or more reflecting surfaces and, more particularly, to a Cassegrainian antenna comprising a main reflector, a doubly curved subreflector, and a doubly curved focal surface on which feeds are appropriately disposed. Subreflector parameters are chosen such that a predetermined amount of distortion is introduced which transforms a three-dimensional, non-rectangular, matrix as might be seen in the far field of the antenna as, for example, an actual view of latitudinal and longitudinal lines of earth as seen from equatorial orbit into a substantially rectangular matrix on a doubly curved focal surface of the antenna.
2. Description of the Prior Art
Except for possibly the boresight beam of an antenna, an antenna beam generally will suffer from some sort of aberration if its feedhorn is located away from the geometrical focus so that a radiated planar wavefront is not produced. This is particularly true in a multibeam antenna. However, antennas have been previously devised to correct for certain aberrations which have been found to exist.
U.S. Pat. No. 3,146,451 issued to R. L. Sternberg on Aug. 25, 1964 relates to a microwave dielectric lens for focusing microwave energy emanating from a plurality of off-axis focal points into respective collimated beams angularly oriented relative to the lens axis. In this regard also see U.S. Pat. No. 3,737,909 issued to H. E. Bartlett et al on June 5, 1973.
Other antenna system arrangements are known which use subreflectors and the positioning of feedhorns to compensate for some aberrations normally produced by such antenna systems. In this regard see, for instance U.S. Pat. Nos. 3,688,311 issued to J. Salmon on Aug. 29, 1972; 3,792,480 issued to R. Graham on Feb. 12, 1974; and 3,821,746 issued to M. Mizusawa et al on June 28, 1974.
U.S. Pat. No. 3,828,352 issued to S. Drabowitch et al on Aug. 6, 1974 relates to microwave antennas including a toroidal reflector designed to reduce spherical aberration. The patented antenna structure comprises a first and a second toroidal reflector centered on a common axis of rotation, each reflector having a surface which is concave toward that common axis and has a vertex located in a common equatorial plane perpendicular thereto.
U.S. Pat. No. 3,922,682 issued to G. Hyde on Nov. 25, 1975 relates to an aberration correcting subreflector for a toroidal reflector antenna. More particularly, an aberration correcting subreflector has a specific shape which depends on the specific geometry of the main toroidal reflector. The actual design is achieved by computing points for the surface of the subreflector such that all rays focus at a single point and that all pathlengths from a reference plane to the point of focus are constant and equal to a desired reference pathlength. The Hyde subreflector, however, (a) only corrects for on-axis aberration of the torus (similar to spherical aberration), (b) only compensates for aberrations when positioned in the far field of the feed, and (c) can be used to produce offset beams in only one plane.
It was found that the dominant aberration introduced in an off-axis beam from a dual reflector or Cassegrainian antennas is astigmatism, which aberration was corrected by the arrangement disclosed in U.S. Pat. No. 4,145,695 issued to M. J. Gans on Mar. 20, 1979, and discussed in the article "Broadband Astigmatic Compensation" by T. Chu in AP-S International Symposium, 1981, Vol. 1, Los Angeles, California, at pp. 131-134.
Although the above-described techniques have compensated for some aberrations found in antennas, none have compensated for, or introduced, for example, barrel distortion for converting, for example, an actual view of the curved latitudinal and longitudinal lines of a celestial body as seen in the far field of a satellite antenna in orbit around the celestial body into a substantially rectangular matrix on the focal surface of the antenna. Therefore, a problem remaining in the prior art is to provide an antenna arrangement which compensates for or introduces a predetermined amount of distortion as, for example, barrel distortion.